Obesity and high-fat diets (HFD) increase the risk for disorders of skeletal muscle function including anabolic resistance, which is an inability to increase protein synthesis in response to feeding or growth cues. Anabolic resistance leads to the attenuation of muscle growth under conditions of increased loading, such as recovery from disuse and functional overload. The objective of this application is to examine the mechanisms by which a HFD and/or obesity reduces load-induced muscle growth in both male and female mice. Our recent findings reveal that in mice fed a HFD muscle growth in response to increased loading is significantly attenuated. These results have led to the formulation of our central hypothesis: the development of anabolic resistance in diet-induced obesity is related to an accumulation of intramuscular lipids and increased fatty acid intermediates leading to an increase in oxidative and endoplasmic reticulum (ER) stress, which inhibits anabolic signaling in response to increased loading. To address our hypothesis, we propose three specific aims that take advantage of unique differences in three mouse strains (C57BL/6J, MuRF1-/-, and ob/ob) and two load-induced growth models: functional overload and hind limb unloading/reloading. State of the art imaging techniques (7-T MRI, laser trapping Raman spectroscopy, and coherent anti-Stokes Raman spectroscopy) will be used to measure intramuscular lipid content and lipid droplet size, number, and composition. In Aim 1 we will test the hypothesis that increasing duration on a diet high in saturated fats, and not adiposity, leads to the development of anabolic resistance in male and female mice. In Aim 2 we will test the hypothesis that a HFD leads to an increase in intramyocellular lipids and the accumulation of fatty acid intermediates causing the development of anabolic resistance. In Aim 3 we will test the hypothesis that MuRF1-/- mice on a HFD have reduced storage of intramuscular lipids and a decrease in fatty acid intermediates, which prevents the development of anabolic resistance. This application is significant because it will lead to major advances in our understanding of the effects of obesity and diets high in saturated fats on the ability of muscle to respond to growth cues, which is clinically important because anabolic resistance leads to decreased mobility and independence, increases the likelihood of injuries and falls, and further increases the risk for diabetes mellitus and cardiovascular disease.